Pseudocapacitive Charge Storage in Electrochromic Transition-Metal Oxide Thin Films

Abstract

Electrochromic pseudocapacitive transition-metal oxide materials, such as tungsten oxide, which combine fast response, high energy density, and optical effects, can play a significant role as energy storage materials. Here we investigate the electrochemical kinetics of thin films of tungsten oxide, which turns transparent to sky-blue in color in the lithiated state due to the reduction of W6+ to W5+. We investigated the charge density, charge transfer, ion diffusion, and interfacial behavior upon Li+ insertion/de-insertion in WO3. The pseudocapacitive and electrical double layer mechanism of the electrochromic thin film was differentiated based on the power-law. Faradaic diffusion-controlled process dominates over the surface capacitive behavior at scan rates below 40 mV/s. These films exhibit areal charge density of around 100 mC/cm2 and a capacitance of 80 mF/cm2, which are superior to most comparable electrochromic materials and supercapacitors. This work reports combining electrochromics and energy storage properties and provides a fundamental understanding of pseudocapacitive and electrochromic mechanisms in WO3.

Aqueous/non-aqueous electrolyte tradeoffs in charge transfer and electrochromics of pseudocapacitive oxide films

“…The electrical double layer can be indicated through the square-like waterfall shape of the 3D plot, while the capacitance increase with potential reflects the ion-diffusion controlled process. 32 For LiClO 4 , the capacitance nearly immediately falls off with the bleaching potential at −0.15 V even at low frequency. In the KOH, the low-frequency capacitance gradually decreases through the applied potentials and doesn't fully deplete as the PC-LiClO 4 , even with the applied potential of −0.4 V. The slower charge release is due to the ion-diffusion controlled process.…”

“…The linear relationship of i ( V )/ v 1/2 vs. v 1/2 enables us to obtain the k 1 and k 2 from the slope and the y -axis intercept, respectively, at every single potential. 32 Firstly presented are the classical CV curves between 20 mV s −1 and 150 mV s −1 scan rates with NiO in Li-PC electrolyte as the demonstration model ( Fig. 9a ).…”

Enhancement of Electrochemical Nitrogen Reduction Activity and Suppression of Hydrogen Evolution Reaction for Transition Metal Oxide Catalysts: The Role of Proton Intercalation and Heteroatom Doping

“…18−20 In fact, electrochemically induced interstitial proton intercalation/deintercalation (xH + + xe − + TMO ↔ H x TMO) has been the working principle behind their applications in supercapacitors and electrochromic-based smart windows. 19,21 Associated with proton intercalation, the injected electrons would occupy the lowest empty states at the bottom of the conduction band of TMOs. 22 Since the d orbital of the transition metal centers is the major contributor to the bottom of conduction band of TMOs, intercalation results in electron population of the d orbital of the transition metal centers of TMOs and upshift of Fermi level (E F ).…”

“…Besides OVs, heteroatom dopants, and the above-mentioned interfacial electric fields, the electronic structure of TMOs can also be modified by in situ proton intercalation, which has largely been overlooked in eNRR studies. − For example, WO 3 readily undergoes bulk hydrogen intercalation in a cathodic electrochemical environment. − In fact, electrochemically induced interstitial proton intercalation/deintercalation ( x H + + x e – + TMO ↔ H x TMO) has been the working principle behind their applications in supercapacitors and electrochromic-based smart windows. , Associated with proton intercalation, the injected electrons would occupy the lowest empty states at the bottom of the conduction band of TMOs . Since the d orbital of the transition metal centers is the major contributor to the bottom of conduction band of TMOs, intercalation results in electron population of the d orbital of the transition metal centers of TMOs and upshift of Fermi level ( E F ).…”